Clarify Redis post upgrade instruction
Clarify Redis post upgrade instruction

After internal discussion, the team decided it is required to point out that flushall is only applicable if Redis is used solely for ThingsBoard.
volodymyr-babak/thingsboard.github.io,thingsboard/thingsboard.github.io,thingsboard/thingsboard.github.io,volodymyr-babak/thingsboard.github.io,volodymyr-babak/thingsboard.github.io,thingsboard/thingsboard.github.io,volodymyr-babak/thingsboard.github.io,thingsboard/thingsboard.github.io